3
Irradiance and Lipid Production in
Natural Algal Populations
Bruce C. Wainman, Ralph E.H. Smith, Hakumat Rai, and
John A. Furgal
3.1. Introduction
Lipids are important to aquatic ecosystems, as essential dietary components for
animals (including some of economic importance in both wild and cultured food
production) (Olsen, this volume), as vectors for movement of hydrophobic materials (including many important contaminants), and as the proximate agents of
toxicity in a variety of organisms (Landrum and Fisher, this volume). Microalgae,
including phytoplankton and attached forms such as ice algae and periphyton, are
major producers of aquatic lipids. A substantial body of measurements of lipid
synthesis by natural populations of microalgae has developed, thanks largely to
the relative ease with which 14C and simple chemical extraction protocols can be
applied to measure the intracellular allocation of recent photosynthate (Morris,
1981; Morris et aI., 1974). In practice, the term photosynthate here refers to
carbon incorporated (and therefore labeled) within the span of typical primary
production experiments (usually 4-24 hours). Such measurements have revealed
substantial variation in the synthesis and relative allocation of photosynthate to
lipids, which may be related to environmental and taxonomic factors (Madariaga,
1992; Wainman and Lean, 1992).
In culture, deficiency of nitrogen, phosphorus, or silicon is an effective stimulus
to increased lipid synthesis in many species of microalgae (Lombardi and Wangersky, 1991; Parrish and Wangersky, 1987; Taguchi et aI., 1987; Shifrin and
Chisholm, 1981). There is evidence from the field for significant increases in lipid
synthesis in situations of depleted nutrient supplies (Smith and D'Souza, 1993;
Palmisano et aI., 1988; Parrish, 1987). Possibly more important than the influence
of nutrients, however, is that of light. The enzyme acetyl CoA carboxylase
catalyzes the first committed step of fatty acid synthesis and is both strongly light
regulated and rate-limiting to fatty acid synthesis in the chloroplasts of higher
plants (Post-Beitenmiller et aI., 1992; Gurr and Harwood, 1991; Harwood, 1988).
De novo fatty acid synthesis in plants also depends largely on NADPH generated
in the light reactions of photosynthesis. It is then no surprise that the lipid synthesis rate and the relative allocation of photosynthate to lipid both tend to increase with incubation irradiance (Wainman and Lean, 1992; Rivkin, 1989; Cuhel
45
Irradiance and Lipid Production in
Natural Algal Populations
Bruce C. Wainman, Ralph E.H. Smith, Hakumat Rai, and
John A. Furgal
3.1. Introduction
Lipids are important to aquatic ecosystems, as essential dietary components for
animals (including some of economic importance in both wild and cultured food
production) (Olsen, this volume), as vectors for movement of hydrophobic materials (including many important contaminants), and as the proximate agents of
toxicity in a variety of organisms (Landrum and Fisher, this volume). Microalgae,
including phytoplankton and attached forms such as ice algae and periphyton, are
major producers of aquatic lipids. A substantial body of measurements of lipid
synthesis by natural populations of microalgae has developed, thanks largely to
the relative ease with which 14C and simple chemical extraction protocols can be
applied to measure the intracellular allocation of recent photosynthate (Morris,
1981; Morris et aI., 1974). In practice, the term photosynthate here refers to
carbon incorporated (and therefore labeled) within the span of typical primary
production experiments (usually 4-24 hours). Such measurements have revealed
substantial variation in the synthesis and relative allocation of photosynthate to
lipids, which may be related to environmental and taxonomic factors (Madariaga,
1992; Wainman and Lean, 1992).
In culture, deficiency of nitrogen, phosphorus, or silicon is an effective stimulus
to increased lipid synthesis in many species of microalgae (Lombardi and Wangersky, 1991; Parrish and Wangersky, 1987; Taguchi et aI., 1987; Shifrin and
Chisholm, 1981). There is evidence from the field for significant increases in lipid
synthesis in situations of depleted nutrient supplies (Smith and D'Souza, 1993;
Palmisano et aI., 1988; Parrish, 1987). Possibly more important than the influence
of nutrients, however, is that of light. The enzyme acetyl CoA carboxylase
catalyzes the first committed step of fatty acid synthesis and is both strongly light
regulated and rate-limiting to fatty acid synthesis in the chloroplasts of higher
plants (Post-Beitenmiller et aI., 1992; Gurr and Harwood, 1991; Harwood, 1988).
De novo fatty acid synthesis in plants also depends largely on NADPH generated
in the light reactions of photosynthesis. It is then no surprise that the lipid synthesis rate and the relative allocation of photosynthate to lipid both tend to increase with incubation irradiance (Wainman and Lean, 1992; Rivkin, 1989; Cuhel
45